Powder compositions for heat sensitive substrates

ABSTRACT

The invention is directed to a powder coating composition comprising a first component (A) and a second component (B) that is dry-blended or melt blended with the first component (A). The first component (A) comprises at least one polyepoxy resin and at least one catalyst. The second component (B) comprises at least one carboxylic acid functional acrylic resin. The polyepoxy resins include non-crystalline polyepoxy resins, crystalline polyepoxy resins, and mixtures thereof. The powder coating compositions can be applied to heat sensitive substrates and cured at lower temperature or for shorter time to provide finishes having controllable gloss and/or high hardness.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of Application No.09/579,621, filed May 26, 2000, now pending.

FIELD OF THE INVENTION

[0002] The invention relates to new thermosetting powder coatingcompositions. Specifically, the invention relates to new acid functionalacrylic/polyepoxy powder coating compositions. More specifically, theinvention relates to new acid functional acrylic/polyepoxy powdercoating compositions for coatings on heat sensitive substrates such aswood substrates to produce thereon by a low temperature cure or a fastcure at higher temperature a finish with high hardness and/orcontrollable gloss.

BACKGROUND OF THE INVENTION

[0003] It is well known that powder coating is a low polluting, costeffective method to provide high quality finishes on substrates such asmetal substrates and therefore, is used more and more to replace liquidfinishing in various industries. Traditionally, powder coatings are usedto coat metal objects, e.g., in the automotive and appliance industries.Metal objects are very good electric conductors and can be heated tohigh temperature. Thus, conventional powder coating compositions areeasily applied onto metal substrates by any of the electrostaticdeposition techniques known in the art to form a fine finish. The coatedmetal substrates are normally cured at elevated temperatures rangingfrom 300° F. to 450° F. Since high temperatures do not significantlydistort metal substrates, heat sensitivity of the work piece isgenerally not a concern. There has been well developed in the art powdercoating compositions that are cured at high temperatures to provide highperformance coatings on metal substrates.

[0004] More recently, efforts have been made to apply powder coatings tonon-metal substrates, known as heat sensitive substrates, e.g.,plastics, pre-assembled products and wood products. Examples of woodproducts include household furniture, office furniture, and constructioncomponents such as doors, siding, widow frames, shelving and molding.While some plastic articles may be manufactured with the appropriatecolor and may not need coatings, some other plastic products andespecially many wood products must be coated for decorative orprotective purpose or both. Although some success has been achieved incoating non-metal substrates, problems remain, especially with coatingson wood substrates due to the distinct natural difference between metalsubstrates and wood substrates.

[0005] Naturally, wood substrates contain more moisture and volatilecompounds. The moisture and the volatile compounds are readily releasedas gasses during the relatively long cure at high temperature associatedwith traditional powder coatings on metal substrates. This outgassingdisrupts the surface of the coating material, resulting in pinholes andother defects in the coating surface, thereby damaging the appearance ofthe coating. Therefore, conventional cure at high temperature for arelatively long time is not usable for coating powders on woodsubstrates. Presently, one approach to avoid pinhole and defects in thecoating surface is to provide a textured coating having, e.g., a grainytexture, or a coating with an orange peel appearance that mask theunderlying non-uniformity of the substrate. Another approach toeliminate outgassing is to develop a low temperature cure coating powdersystem to lower the cure temperature and/or to shorten the cure time.However, the lower the cure temperature and/or the shorter the cure timeare, the more difficult it is to achieve high performance. Likewise, thelower the cure temperature and/or the shorter the cure time are, themore difficult it is to control gloss, especially to achieveaesthetically desired low gloss. Furthermore, the lower the curetemperature and/or the shorter the cure time are, the more difficult itis to process the powder coatings as traditional process temperaturesare in the same range as the targeted cure temperatures. Achieving highperformance coatings as defined by toughness and durability tests likehardness, scratch, mar, chemical resistance and impact can bechallenging with traditional coating powders cured at high temperaturesfor a longer time. Again the challenge becomes greater as the curetemperature is lowered or the cure time is shortened.

[0006] Various powder coating compositions used on either or both metaland wood substrates in the relevant art are described in the patentliterature.

[0007] U.S. Pat. No. 6,022,927 discloses an acrylic powder coatingcomposition used on metal and wood substrates. The composition containsbasically an acid functional acrylic resin; an adduct of an acidfunctional polyester and a polyepoxy (e.g., trigylcidylisocyanurate(TGIC)); and a curing agent that is also TGIC to provide orange peeltextured surfaces on both metal and wood substrates. It is specificallytaught that a coating powder prepared by simply mixing the carboxylfunctional acrylic resin, the curing agent (TGIC) and the polyester doesnot provide a coating with desired physical properties. Only if thecuring agent (TGIC) and the polyester are adducted prior to forming thecoating powder, can the coating with desired properties be produced. Thecure temperature may be reduced to 300° F. or below upon addition of acure catalyst. 60° gloss tested on metal substrate is higher than 75.Pencil Hardness of the coating surface, tested on metal panel, is from Fto 2H.

[0008] U.S. Pat. No. 5,907,020 discloses a thermosetting powder coatingsystem, based on a mixture of an extrudate of a catalyzed self-curingepoxy resin and a low temperature curing agent. The epoxy resin is firstextruded with a catalyst, then the extrudate is mixed with a lowtemperature curing agent to form the coating powders. The lowtemperature curing agent is an epoxy adduct of an imidazole catalyst oran epoxy adduct of an aliphatic polyamine, to provide low gloss coatingson metal and wood substrates. The cure temperature may be 290° F. orbelow. 60° gloss tested on steel panel varies from 15 to 80.

[0009] U.S. Pat. No. 5,721,052 discloses a thermosetting powder coatingcomposition based on Bisphenol A type of epoxies, an imidazole catalystand a texturing agent, for forming a grainy textured finish on bothmetal and wood substrates. The cure temperature may be about 350° F. orbelow. 60° gloss tested on metal panel is below 6. Pencil Hardness,tested on metal panels, is from H to 2H.

[0010] All the powder coating compositions disclosed, however, do notprovide coatings on heat sensitive substrates, especially woodsubstrates with the advantageous combination of low cure or fast cure,controllable gloss, and high hardness.

[0011] Therefore, there is a need for a new powder coating compositionthat can be coated by means of traditional powder coating applicationmethods on heat sensitive substrates, especially wood substrates andcured either at relatively lower temperatures for relatively longer timeor at relatively higher temperatures for relatively shorter time toproduce a finish with controllable gloss and very high performance suchas hardness without damaging the substrates.

SUMMARY OF THE INVENTION

[0012] The invention provides a new powder coating composition inparticulate form consisting essentially of a mixture of:

[0013] a). about 10% to about 90% by weight, relative to (a) plus (b),of carboxylic acid functional acrylic resin(s);

[0014] b). about 10% to about 90% by weight, relative to (a) plus (b),of polyepoxy resin(s);

[0015] c). a catalyst in an amount sufficient to cure the composition ata temperature about 300° F or below for about 30 minutes or less; and

[0016] d). optionally, a flexibilizing agent.

[0017] In another aspect, the invention provides a new powder coatingcomposition comprising:

[0018] a). about 10% to about 90% by weight, relative to (a) plus (b),of carboxylic acid functional acrylic resin(s);

[0019] b). about 10% to about 90% by weight, relative to (a) plus (b),of polyepoxy resin(s); and

[0020] c). a catalyst in an amount sufficient to cure the composition ata temperature about 300° F. or below for about 30 minutes or less,wherein the composition provides a finish having a pencil hardness of atleast 3H.

[0021] In yet another aspect, the invention provides a new powdercoating composition comprising:

[0022] a). about 10% to about 90% by weight, relative to (a) plus (b),of carboxylic acid functional acrylic resin(s);

[0023] b). about 10% to about 90% by weight, relative to (a) plus (b),of polyepoxy resin(s);

[0024] c). a first catalyst which functions primarily as an epoxyhomopolymerization catalyst; and

[0025] d). a second catalyst which functions primarily as anepoxy/carboxylic acid reaction catalyst.

[0026] In yet another aspect, the invention features a powder coatingcomposition comprising:

[0027] A). about 50% to about 99% by weight of a first componentcomprising

[0028] a-1) at least one polyepoxy resin; and

[0029] a-2) at least one catalyst, and

[0030] B). about 1% to about 50% by weight of a second componentdry-blended with said first component (A), comprising at least onecarboxylic acid functional acrylic resin.

[0031] In yet another aspect, the invention features a powder coatingcomposition comprising:

[0032] a). about 1% to about 50% by weight of at least one carboxylicacid functional acrylic resin;

[0033] b). about 50% to about 99% by weight of more than one polyepoxyresin, of which from about 2% to about 15% by weight, based on the totalepoxy resins, being at least one crystalline polyepoxy resin; and

[0034] c). a catalyst in an effective amount to cure said composition.

[0035] The new powder coating compositions of the invention are readilyapplied, e.g., by means of electrostatic deposition or fluidized bedmethods, to various substrates, especially heat sensitive substrates,e.g., plastic or wood substrates to provide finishes having controllablegloss and high performance including very high hardness.

[0036] The coatings of the invention may be cured at a low curetemperature of no greater than about 300° F. for no longer than about 30minutes to form coating films on substrates, especially wood substrates,thereby, diminishing outgassing from the substrates without damaging thesubstrates. The cured coatings of the invention may produce 60° gloss offrom about 2 to about 95 and pencil hardness of greater than F,preferably greater than 3H.

[0037] Particularly, the powder coating compositions of the inventionare readily coated by electrostatic spray and cured on heat sensitivesubstrates, especially wood substrates at a temperature of about 280° F.or below for about 20 minutes or less, to produce a finish having anaesthetically acceptable 60° low gloss of from about 5 to about 60,preferably, of from about 10 to about 40, and pencil hardness of greaterthan H, preferably greater than 3H.

[0038] The coatings of the invention may also be cured at a fast curetemperature of higher than about 300° F., preferably higher than about325° F. for a period of no greater than about 10 minutes, preferably nogreater than about 5 minutes to form coating films on substrates,especially wood substrates, while still exhibiting the required physicaland performance properties such as controllable gloss and/or pencilhardness.

[0039] In yet another aspect, the invention provides an articlecomprising a heat sensitive substrate coated on at least one surface ofthe substrate with any of the aforesaid new powder coating compositionsof the invention and cured either at a temperature of no greater thanabout 300° F. for a period of no greater than about 30 minutes or at atemperature of greater than about 300° F., preferably greater than about325° F. for a period of no greater than about 10 minutes to produce adecorative and/or protective finish with controllable 60° gloss of fromabout 2 to about 95 and controllable performance regarding chemicalresistance, hardness, etc., depending on application.

[0040] In particular, the article of the invention comprises a woodsubstrate coated on at least one of the surfaces of the substrate withany of the aforesaid new powder coating compositions of the inventionand cured either at a temperature of about 300° F. or below for about 30minutes or less or at a temperature of greater than about 300° F.,preferably greater than about 325° F. for a period of no greater thanabout 10 minutes to produce a finish having 60° gloss of from about 5 toabout 60, preferably from about 10 to about 40, and pencil hardness ofgreater than F, preferably, greater than H, and more preferably no lessthan 3H.

DETAILED DESCRIPTION OF THE INVENTION

[0041] For the purpose of the invention, the term “wood” is defined asincluding natural wood and engineered wood such as plywood,particleboard, oriented strand board, hardboard, medium densityfiberboard, and the like. The particleboard may be standard or treatedto enhance its electrical conductivity. Wood, pre-coated with aconductive liquid coating composition and cured, may also be used as asubstrate for the purpose of the invention. Wood having moisture contentof from about 3 to about 10% by weight is preferred.

[0042] For the purpose of the invention, “reduced or low curetemperature” refers to a substrate surface temperature, also known aspart temperature, that is about 300° F. or below to achieve desiredperformance, as compared to the traditional high cure temperature ofhigher than 300° F. to about 450° F.

[0043] For the purpose of the invention, “fast cure” refers to a curethat is carried out at a fast cure temperature, which refers to asubstrate surface temperature, also known as part temperature, that canbe as high as the traditional cure temperature, but the cure time isshorter than that required for curing the traditional powder coatingsystems in order to achieve the desired performance.

[0044] The thermosetting powder coating compositions of the inventioninclude carboxylic acid functional acrylic resin(s). The carboxylic acidfunctional acrylic resins useful in the invention are formed fromtypical acrylic monomers known in the art, such as acrylic acid ormethacrylic acid; acrylic acid derivatives such as methyl acrylate,ethyl acrylate, propyl acrylate, isopropyl acrylate, butyl acrylate,isobutyl acrylate, 2-ethylhexyl acrylate, isooctylacrylate, dodecylacrylate, stearyl acrylate, cyclohexyl acrylate, benzyl acrylate,hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate,1,4-butanediol monoacrylate and dimethylaminoethyl acrylate; methacrylicacid derivatives such as methyl methacrylate, ethyl methacrylate, propylmethacrylate, isopropyl methacrylate, butyl methacrylate, isobutylmethacrylate, isooctylmethacrylate, 2-ethylhexyl methacrylate, dodecylmethacrylate, steary methacrylate, cyclohexyl methacrylate, benzylmethacrylate, phenyl methacrylate, hydroxyethyl methacrylate,hydroxylpropyl methacrylate, hydroxybutyl methacrylate, 1,4-butanediolmonomethacrylate and dimethylaminoethyl methacrylate; etc. Such monomersmay be used each separately or in the combination of two or more. Othermonomers such as styrene, substituted styrene, or vinyl toluene, etc.may also be incorporated. Commercially available examples of thecarboxylic acid functional acrylic resins include, e.g., those under thetrademark designation SCX™ acrylic resins by S. C. Johnson.

[0045] According to the invention, carboxylic acid functional acrylicresins may be used separately or in combination of two or more in thepowder coating composition of the invention. In some embodiments, thecomposition may comprise about 10% to about 90% by weight, preferably,about 10% to about 60% by weight, and most preferably, about 10% toabout 40% by weight of the carboxylic acid functional acrylic resin(s),based on the total weight of resins used in the powder coatingcomposition of the invention. In some embodiments wherein the carboxylicacid functional acrylic resin(s) are dry-blended with the othercomponents, the amount of the carboxylic acid functional acrylicresin(s) is preferably from about 1% to about 50%, preferably from about2% to about 20% by weight, based on the total weight of resins used inthe powder coating composition of the invention.

[0046] The powder coating composition of the invention also includespolyepoxy resin(s). A wide variety of polyepoxy resins includingnon-crystalline polyepoxy resins and crystalline polyepoxy resins may beused in the powder coating compositions of the invention. Typically, thepolyepoxy resins should have at least two epoxy groups per molecule,including saturated or unsaturated, aliphatic, cycloaliphatic orheterocyclic compounds and may be substituted with substituents such ashalogen atoms, alkyl groups, ether groups and the like. Suitablepolyepoxy resins include glycidyl ethers of aromatic and aliphaticpolyols, cycloaliphatic polyepoxides, epoxy-functional acrylic resins,heterocyclic polyepoxides, glycidyl esters of aromatic and aliphaticpolycarboxylic acids, glycidyl polyamines and ether amines, and mixturesthereof. For example, the polyepoxy resins may be solid epoxy resinsbased on bisphenol A (2,2′-bis(p-hydroxyphenyl)propane) andepichlorohydrin, i.e., the diglycidyl ether of bisphenol A and higheraddition products thereof. The polyepoxy resins may include solid epoxyresins based on bisphenol F (4,4′-dihydroxydiphenylmethane), saturatedbisphenol A (2,2′bis(4-hydroxycyclohexyl)propane) and epichlorohydrinand higher addition products. Also included are epoxy resins made by thereaction of epichlorohydrin with a novolac resin. A novolac resin is acondensate of a phenol compound with formaldehyde in the presence ofacid catalysts. The phenol compound can be phenol itself, or compoundssuch as cresols, xylenols, resorcinol, naphthols, and the like. Theepoxy resins are available from a wide variety of commercial sourcesunder the trademark designation such as EPON™ or Epikote™ fromResolution Performance Products (Houston, Tex.), Araldite™ from Vantico(Hawthorne, N.Y.) and DER from Dow Chemical Company (Midland, Mich.).Commercially available epoxy resins include Epikote™ 1001, Epikote™1002, Epikote™ 1004, Epikote™ 1007, and Epikote™ 1009 from ResolutionPerformance Products; Epo-Thoto YD-012 from KUKDO Chemical Ind. Co.,Ltd.; DER™ 642 and DER™ 672 from Dow Chemical Co.; Araldite™ 7220,Araldite™ ECN 1235, Araldite™ ECN 1273, and Araldite™ ECN 1280 fromVantico. Other useful epoxy resins include bisphenol S epoxy resins;hydrogenated bisphenol A epoxy resins. Polyepoxy resins based onbisphenol A are preferred from among the polyglycidyl ethers of aromaticpolyols. Useful heterocyclic polyepoxides include 1,3,5-tris(2,3-glycidyl-propyl) 1,3,5-triazine-2,4,6-(1H,3H,5H)-trione known astriglycidyl isocyanurate (TGIC), 1,3,5-tris (2,3-glycidyl-2-methylpropyl) 1,3,5-triazine-2,4,6-(1H,3H,5H)-trione, the diglycidyl ester ofterephthalic acid and the triglycidyl ester of trimellitic acid.Commercially available heterocyclic polyepoxides include Araldite™PT-810 from Vantico and MT-239 from Nissan Chemical; and glycidyl estersinclude PT-910 from Vantico. Other useful crystalline polyepoxidesinclude tetramethylbisphenol diglycidyl ether, bisphenol S diglycidylether, 2,5-di-t-butylbenzene-1,4-diglycidyl ether, hydroquinonediglycidyl ether, 2,5-di-t-butylhydroquinone diglycidyl ether,diglycidyl isophthalate, and epoxypropoxydimethylbenzyl acrylamide.Commercially available crystalline polyepoxy resins include EpoxyResearch Resin RSS-1407 from Resolution Performance Products, EPO-THOTOYDC-1312 from KUKDO Chemical Ind., Co., Ltd. Suitable epoxy functionalacrylic resins may be produced by polymerizing epoxy functionalacrylates alone or in combination with other vinyl monomers, includingother acrylic esters, styrene and substituted styrenes, as specifiedbefore. Examples of epoxy functional acrylate monomers include glycidylacrylate, glycidyl methacrylate, beta-methylglycidyl acrylate,beta-methylglycidyl methacrylate, N-glycidyl acrylic acid amide and thelike, among which glycidyl acrylate and glycidyl methacrylate arepreferred. Examples of commercially available epoxy functional acrylicresins include Fineclad™ A-244-A by Reichhold, Almatex™ PD 7690 byAnderson Co. and GMA 300™ by Estron Chemical Company.

[0047] The polyepoxy resins may be used separately or in combination oftwo or more in the powder coating compositions of the invention. Forexample, one bisphenol A type of polyepoxy resin may be used alone or incombination with other bispenol A type of polyepoxy resin or withnovolac type of polyepoxy resins. Likewise, bisphenol A type ofpolyepoxy resins may be used in combination with triglycidylisocyanurate and/or with epoxy functional acrylic resins. Also,non-crystalline polyepoxy resin(s) may be used in combination withcrystalline polyepoxy resin(s).

[0048] The powder coating composition of the invention may comprise fromabout 10% to about 90% by weight, preferably, from about 40% to about90% by weight, and more preferably, from about 60% to about 90% byweight of the polyepoxy resin(s), based on total weight of resins usedin the powder coating composition of the invention. In the embodimentswherein the polyepoxy resin(s) are dry-blended with the othercomponents, the amount of polyepoxy resin(s) is preferably from about50% to about 99%, more preferably from about 80% to about 98% by weight.In the embodiments wherein the crystalline polyepoxy resin(s) are usedin combination with non-crystalline polyepoxy resin(s), the crystallinepolyepoxy resin(s) may be present in an amount of from about 2% to about15% by weight, based on the total weight of the polyepoxy resins.

[0049] The powder coating compositions of the invention may also includecatalyst(s) that allow the powder coating to cure either at a low curetemperature of no greater than about 300° F., preferably, no greaterthan about 280° F., or at a fast cure temperature for as short time aspossible to minimize the temperature build up within the substrates.Examples of the useful catalysts may include imidazoles, imidazole/epoxyadducts, tertiary amines, imidazolines, imidazoline salts of mono- ordi-carboxylic acids, tetraalkylammonium salts, phosphonium salts, tincatalysts e.g., stannous octoate, and mixtures thereof. Examples ofimidazoles may include substituted and unsubstituted imidazoles, such asimidazole, 2-methylimidazole, and 2-phenylimidazole. Examples ofimidazole/epoxy adducts can be commercially available under the tradedesignation EPON P-101 from Resolution Performance Products and underthe trade designation XU HT 261 from Vantico. Examples oftetraalkylammonium salts may include tetramethylammonium bromides,tetramethylammonium iodides, tetramethylammonium chlorides,myrystyltrimethylammonium bromides, myrystyltrimethylammonium iodides,myrystyltrimethylammonium chlorides, and the like. Examples ofphosphonium salts may include ethyltriphenylphosphonium bromides,ethyltriphenylphosphonium iodides and ethyltriphenylphosphoniumchlorides, and the like. Examples of tertiary amines may includeN,N-dimethylcyclohexylamine, N,N-dimethylaniline, N-methylmorpholine,N,N′-dimethylpiperazine, 2,2,6,6,-tetramethyl-4-dimethylaminopiperidine,N,N-dimethyloctadecylamine, N,N-dimethylhexadecylamine,1,8-diazabicyclo[5.4.0]undec-7-ene,N,N,N′,N′-tetramethylhexamethylenediamine,N,N,N′,N′,N″-pentamethyldiethylenetriamine, triethylenediamine andbenzyldimethylamine. Examples of imidazolines include substituted andunsubstituted imidazolines, such as 2-phenylimidazoline. Imidazolinesalts of mono- or di-carboxylic acids are derived from imidazoline-basedcompounds and mono- or di-carboxylic acids. Suitable monocarboxylicacids and dicarboxylic acids have one or two carboxylic acid groups(—COOH) per molecule respectively. They include aromatic and aliphatic(saturated and unsaturated) acids and combinations thereof (i.e.,araliphatic). Typical monocarboxylic acids or dicarboxylic acidsinclude, but are not limited to, terephthalic acid, isophthalic acid,phthalic acid, benzoic acid, azelaic acid, adipic acid, succinic acid,glutaric acid, pimelic acid, suberic acid, sebacic acid,decanedicarboxylic acid, dodecandicarboxylic acid, acetic acid,2-ethylhexanoic acid and maleic acid. Preferred mono- and di-carboxylicacids are adipic acid, sebacic acid and dodecanedioic acid.

[0050] Imidazoline-based compound such as a 2-imidazoline compound maybe represented by the following general structure:

[0051] wherein R¹ is hydrogen, an aromatic group, an aliphatic group(saturated or unsaturated), cycloaliphatic or an araliphatic group,preferably having about 1-12 carbon atoms. Examples of R¹ includephenyl, 2-chlorophenyl, 2-hydroxyphenyl, 4-chlorophenyl, 4-methylphenyl, and the like, alkyl, such as methyl, undecyl and the like,aralkyl, such as benzyl and the like, or hydrogen. R² is hydrogen or analkyl group, preferably having about 1 to 12 carbon atoms, such asmethyl and the like.

[0052] Typically, 2-imidazolines include 2-phenyl-2-imidazoline,2-(2-hydroxyphenyl)-2-imidazoline, 2-(2-chlorophenyl)-2-imidazoline,2-(4-chlorophenyl)-2-imidazoline, 2-(4-methylphenyl)-2-imidazoline,2-n-undecyl-2-imidazoline, 2-benzyl-2-imidazoline,4,4-dimethyl-2-imidazoline, and 2-methyl-2-imidazoline. The Imidazolinesalts of mono- or di-carboxylic acids and their preparations aredescribed, for example, in GB 2,312,897, which is incorporated herein byreference.

[0053] The powder coating composition of the invention can be formulatedto include at least one catalyst in an amount sufficient to cure thecomposition. Preferably, the catalyst is present in an amount sufficientto cure the composition either at a low cure temperature of no greaterthan about 300° F. for a relatively longer period, such as about 30minutes or less, or at a fast cure of higher than about 300° F.,preferably higher than about 325° F. for a period of no longer thanabout 10 minutes, or higher than about 350° F. for a period of no longerthan about 5 minutes, or higher than about 400° F. for a period of nolonger than about 3 minutes. Preferably, the catalyst is present in anamount of from about 0.1 to about 10.0 pphr (parts per hundred resin),more preferably, from about 0.5 to about 5 pphr, and most preferably,from about 1.0 to about 4.0 pphr.

[0054] In another aspect of the invention, the powder coatingcompositions may comprise a first catalyst to function primarily as anepoxy homopolymerization catalyst and a second catalyst to functionprimarily as an epoxy/carboxylic acid reaction catalyst. Examples ofuseful first catalyst may include imidazoles, imidazole/epoxy adducts,tertiary amines, and mixtures thereof. Examples of useful second mayinclude imidazolines, imidazoline salts of mono- or di-carboxylic acids,tertiary amines, tetraalkylammonium salts, phosphonium salts, andmixtures thereof.

[0055] Although not bounded by any theory, it is believed that animidazole adducts to epoxy resin by opening an epoxy ring. The adductedimidazole acts as a catalyst that facilitates further epoxy ringopening, thereby promoting the homopolymerization of the epoxide groups.On the other hand, an imidazoline based catalyst may promote additionreactions between the epoxide groups and the carboxylic acid groups,thereby further reducing the cure temperature.

[0056] The powder coating compositions of the invention can beformulated such that, by adjusting the amounts and ratios of thecatalysts, the coatings can be cured either at a low cure temperaturesuch as at about 300° F. or below, preferably at about 280° F. or below,or at a fast cure such as higher than about 300° F., preferably, no lessthan about 350° F. for as short time as possible to achieve the desiredgloss and high performance, e.g., high hardness.

[0057] Optionally, the powder coating compositions of the invention maycontain other additives that are common to powder coatings. Theseadditives include, without limitation, fillers, slip additives,pigments, dyes, UV stabilizers, antioxidants, fluidizing agents, flowcontrol agents, degassing agents, flexibilizing agents, texturingagents, etc. and mixtures thereof. For example, flow control agents suchas an acrylic compound, e.g., Modaflow 2000 or Resiflow P-67, or asilicon compound are known in the coating or powder coating art and aregenerally incorporated into the powder coating compositions to improvethe surface tension, thereby facilitating the flow of the polymers asthey are melted to provide a smoother finish of the final solidcoatings. Flexibilizing agents such as solid plasticizers, core/shellmaterials, rubber, hydroxyl or acid functional polyester, styrene/maleicanhydride and polyanhydride resins are used to provide a finish withmore flexibility. Examples of useful plasticizers may include sucrosebenzoate, pentaerythritol tetrabenzoate and cyclohexanedimethanoldibenzoate. Examples of useful rubbers may include natural and mostsynthetic rubbers, such as styrene-butadiene and acrylonitrile-butadienepolymers. Examples of useful polyesters may include those formed by thecondensation reaction of aliphatic polyols, including cycloaliphaticpolyols, with aliphatic and/or aromatic polycarboxylic acids andanhydrides. Examples of suitable aliphatic polyols may include1,2-ethanediol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol,neopentyl glycol, cyclohexane dimethanol, trimethlyopropene, and thelike. Examples of suitable polycarboxylic acids and anhydrides mayinclude succinic acid, adipic acid, azelaic acid, sebacic acid,terephthalic acid, isophthalic acid, tetrahydrophthalic acid,hexahydrophthalic acid, trimellitic acid, and anhydrides of such acids.The flexibilizer, if needed, may be present up to 50%, preferably, fromabout 2% to about 30% by weight in the composition. Pigments may beadded to give the powder coatings color. Examples of pigments include,but are not limited to, titanium dioxide, carbon black, and organic andinorganic pigments of any available color depending on the application.Special effects additives to produce appearances such as veins,wrinkles, and the like may also be added. The amounts of theseadditional additives depend upon the particular properties of the finalcoating desired.

[0058] The powder coating compositions of the invention may be preparedby conventional techniques employed in the powder coating art.Typically, the components of the powder coating composition are combinedand thoroughly blended together and then, melt blended and extruded in asingle screw or twin screw extruder. The extrudate is immediately cooledand then is ground in a mill, such as a Brinkman mill, a Bantam hammermill, an Alpine Mill or an ACM Mill, and sieved to obtain a powder ofappropriate particle size depending on the application. Although a widerange of the particle sizes is useful for the purpose of the invention,typically, the average particle size is about 5 to about 250 micron.Preferably, the average particle size of the powder coating compositionof the invention is from about 10 microns to about 80 microns, and morepreferably, from about 20 to 40 microns.

[0059] In yet another aspect of the invention, the powder coatingcomposition comprises a dry blend of a first component (A) and a secondcomponent (B). The first component (A) is an extruded blend comprisingat least one polyepoxy resin and at least one catalyst. The secondcomponent (B) comprises at least one carboxylic acid functional acrylicresin. The polyepoxy resin(s) may be present in the powder coatingcomposition of the invention in an amount of from about 50% to about 99%by weight, preferably from about 80% to about 98% by weight. Thecarboxylic acid functional acrylic resin(s) may be present in an amountof from about 1% to about 50% by weight, preferably from about 2% toabout 20% by weight, based on the total weight of the first and thesecond components (A) and (B).

[0060] Optionally, the second component (B) may also comprise a catalystor a combination of more than one catalyst. In addition, the component(A) and/or the component (B) may also contain at least one acidfunctional polyester and/or other additives that are common to powdercoatings. These additives include, without limitation, fillers, slipadditives, pigments, dyes, UV stabilizers, antioxidants, fluidizingagents, flow control agents, degassing agents, flexibilizing agents,texturing agents, etc. and mixtures thereof.

[0061] In yet another aspect, the invention provides a process formaking the dry-blended powder coating composition of the invention. Theprocess comprises melt-blending the ingredients (a-1), (a-2) and otheroptional ingredients to obtain the first component (A), and thendry-blending the first component (A) with a second component (B) toobtain the dry-blended powder coating composition of the invention. Thesecond component (B) may be a carboxylic acid functional acrylic resinthat does not need to be extruded and thus, may be directly added to thefirst component (A) in powder form to obtain the dry-blended powdercoating composition. Alternatively, the second component (B) may beprepared separately by melt-blending the carboxylic acid functionalacrylic resin with other optional ingredients prior to being added tothe first component (A).

[0062] Both the first component (A) and the second component (B) can bein any form such as powder, flakes, chips, prills, etc., prior to dryblending together.

[0063] Grinding and sieving may be performed before or afterdry-blending to obtain the dry-blended powder coating composition ofappropriate particle size.

[0064] In one embodiment, the first component (A) is prepared bycombining the ingredients (a-1), (a-2), and other optional ingredients,if present, and mixing them thoroughly. Then the mixture is extruded inan extruder, such as a single or a twin screw extruder. The extrudate isimmediately cooled and ground in a mill, such as a Brinkman mill, aBantam hammer mill, an Alpine Mill or an ACM Mill, and sieved to obtaina powder of appropriate particle size depending on the application.Then, the second component (B), which is a carboxylic acid functionalacrylic resin that does not need to be extruded, is separately added toand mixed thoroughly with the component (A) to obtain the dry-blendedpowder coating composition of the invention.

[0065] In another embodiment, the second component (B) is prepared bymelt-blending the carboxylic acid functional acrylic resin(s) with otheroptional ingredients including, e.g., catalyst(s), crosslinkingagent(s), acid functional polyester, and/or other additives, if present.The extrudate is ground and sieved to obtain a powder of appropriateparticle size of from about 0.1 micron to about 250 micron, preferably,from about 1 micron to about 100 micron and more preferably, from about5 micron to about 60 micron. Then, two components, each being in powderform, are dry-blended together, i.e., one component, e.g., the secondcomponent (B) is separately added to, or mixed with the first component(A), or vise versa to obtain the dry-blended powder coating compositionof the invention.

[0066] In yet another embodiment, the first component (A) and the secondcomponent (B) are prepared by separately extruding each component withany other optional ingredients. Then the extrudates of the twocomponents are mixed together, ground and sieved to obtain the powdercomposition of appropriate particle size.

[0067] In yet another embodiment, the second component (B) does not needto be extruded but may need to be ground. The component (B) can beseparately added to the extrudate of the first component (A), and then,the two components are ground and sieved to obtain the dry-blendedpowder composition.

[0068] Although a wide range of the particle sizes is useful for thepurpose of the invention, typically, the average particle size of thepowder coating composition of the invention is from about 5 microns toabout 250 microns. Preferably, the average particle size is from about10 microns to about 80 microns, and more preferably, from about 20microns to about 55 microns.

[0069] The powder coating compositions of the invention can be appliedonto at least one of the surfaces of a substrate using any conventionalpowder coating deposition technique, such as electrostatic spray, toobtain smooth and uniform coatings. Curing is achieved by heating thecoated substrate at a temperature for a time sufficient to cure thecomposition. By addition of the catalyst(s), the cure temperature of thepowder coating compositions of the invention can be adjusted to either alow temperature or a high temperature for a short time. Both the lowcure temperature or higher temperature for a shorter cure time limit thesubstrate heat exposure, thereby diminishing the outgassing from thewood substrates that could degrade or damage the substrate integrity aswell as the continuous uniformity of the final finish. Preferably, thecure temperature of the powder coating compositions of the invention iseither at about 300° F. or below, more preferably, at about 280° F. orbelow or at a higher temperature for a shorter period of time. The curetime varies depending on the cure temperature, the nature and thethickness of the substrate. Preferably, at low cure temperature, thecure time may be shorter than 30 minutes, more preferably, shorter thanabout 20 minutes. At a higher cure temperature, such as higher thanabout 325° F. or 350° F., the cure time is preferably shorter than about10 minutes, more preferably, shorter than about 5 minutes.

[0070] The thickness of the cured coatings of the invention variesdepending on the application and performance requirements, butpreferably, ranging from about 1.5 mil to about 8.0 mil.

[0071] The powder coating compositions of the invention may be appliedto various conventional substrates such as metals, e.g., steel, oraluminum; glass; ceramic; carbon-fiber; and graphite. Particularly, thepowder coating compositions of the invention can be applied to heatsensitive substrates such as plastic or fiber-reinforced plasticsubstrates, and especially wood substrates.

[0072] Typically, the lower the cure temperature and/or the shorter thecure time are, the more difficult it is to achieve high performance.Likewise, the lower the cure temperature and/or the shorter the curetime are, the more difficult it is to control gloss, especially toachieve aesthetically desired low gloss. Further, the lower the curetemperature and/or the shorter the cure time are, the more difficult itis to process the powder coatings as traditional process temperaturesare in the same range as the targeted cure temperatures. Achieving highperformance coatings as defined by toughness and durability tests likehardness, scratch, mar, chemical resistance and impact can bechallenging with traditional coating powders cured at high temperatures.Again the challenge becomes greater as the cure temperature is loweredor the cure time is shortened.

[0073] Surprisingly, the powder coating compositions of the inventioncan be formulated to provide the coated surface of the articles with thedesired high performance and other characteristics while cured at a lowtemperature or a higher temperature for a shorter period of time. Forinstance, the powder coating compositions of the invention can beformulated to provide finishes with a wide range of 60° gloss, eitherhigh or low, depending on the application. Surprisingly, by dry-blendingthe components of the composition, the adjustment of the gloss becomesmuch easier. For aesthetically desired low gloss, the powder coatingcompositions of the invention can be formulated, such as by selectingthe resins with low compatibility or by preparing the composition usingdry blending technique, to provide coatings with controllable 60° glossof from about 5 to about 60, preferably, from about 10 to about 40.

[0074] Further, the powder coating compositions of the invention can beformulated to provide finishes with varied hardness, depending on theapplication, such as by changing the degree of cross-linking density, orthe degree of cure, or by selecting resins having different glasstransition temperatures (Tgs). For example, when high hardness such asthat measured by pencil hardness is desired, the powder coatingcompositions of the invention can be formulated using resin(s) with anincreased Tg, or formulated to increase the crosslinking density or thedegree of cure. Alternatively, increasing the cure time at low curetemperature or increasing the cure temperature can increase the degreeof cure. Typically, the composition of the invention can be formulatedto provide coatings with pencil hardness of no less than about 2H, andpreferably, no less than about 3H, depending on the application.

[0075] In addition, the powder coating compositions of the invention canbe formulated to provide coatings on substrates, especially engineeredwood substrates, with aesthetically acceptable smooth appearance as wellas orange peel or textured appearance depending on the application. Forexample, by incorporating small amounts of crystalline polyepoxyresin(s) into the composition, the smoothness of the finish obtained canbe improved. In addition, the process window is significantly widened.

[0076] In yet another aspect, the invention provides an articlecomprising a heat sensitive substrate coated on at least one of thesurfaces of the substrate with any of the aforesaid new powder coatingcompositions and cured either at a low cure temperature or a higher curetemperature for shorter time to produce a decorative and/or protectivefinish with controllable gloss, especially aesthetically desired lowgloss and/or very high hardness.

[0077] Particularly, the article of the invention comprises a heatsensitive substrate, especially wood substrate, coated on at least oneof the surfaces of the substrate with any of the aforesaid new powdercoating compositions and cured either at a low cure temperature of about300° F. or below or at a higher cure temperature, such as higher thanabout 325° F., for a shorter period of time to produce a decorativeand/or protective finish with 60° gloss of from about 2 to about 95,preferably, from about 5 to about 60, and more preferably, from about 10to about 40, and pencil hardness of no less than F, preferably, no lessthan H, and more preferably no less than 3H.

[0078] The invention is further illustrated by the followingnon-limiting examples. It should be understood, however, that manyvariations and modifications may be made while remaining within thescope of the present invention. All components are measured as parts.All the parts, ratios, and percentages specified herein are by weightunless otherwise stated.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

[0079] Testing Method:

[0080] Gloss

[0081] The gloss of the cured powder coating of the invention ismeasured according to the Standard Test Method for Specular Gloss ofASTM D523.

[0082] Pencil Hardness

[0083] The pencil hardness of the cured powder coating of the inventionis measured according to the Standard Test Method for Film Hardness byPencil Test of ASTM D3363.

[0084] MEK resistance

[0085] The MEK resistance of the cured powder coating of the inventionis measured according to the Standard Test Method for Measuring MEKResistance by Solvent Rub of ASTM (Test Method) D4752-98.

EXAMPLES Example 1

[0086] A powder coating composition was prepared by pre-mixing 5.0 partsSCX 843 (acid functional acrylic resin, S.C. Johnson, Wis.), 48.5 partsYD012 (polyepoxy resin, KUKDO Chemical Ind. Co., Ltd. Seoul, Korea), 3.7parts by weight PT810 (TGIC, Vantico), 47.8 parts GMA 300 (epoxyfunctional acrylic resin, Estron Chemical Company, Conn.), 2.5 parts2-phenyl-2-imidazoline/dodecandioic acid disalt, 1.2 parts2-pheyl-imidazole, and 1.5 parts Resiflow P-67 (acrylic resin, flowadditive). The mixture was then melt blended in a twin-screw extruder at500 rpm, extruded, cooled, grounded and then sieved through 100 meshsieve to obtain coating powders.

[0087] Using an electrostatic spray coating method, the above-preparedcoating powders were then applied onto a medium density fiberboard andcured in an oven at about 275° F. for about 10 minutes. The 60° gloss,pencil hardness and MEK resistance of the cured powder coating weretested and the test results are shown in Table I.

Example 2

[0088] A powder coating composition was prepared according to Example 1,except that 10.0 parts SCX 843 was used. The 60° gloss, pencil hardnessand MEK resistance of the cured powder coating were tested and the testresults are shown in Table I.

Example 3

[0089] A powder coating composition was prepared according to Example 1,except that 15.0 parts SCX 843 was used. The 60°° gloss, pencil hardnessand MEK resistance of the cured powder coating were tested and the testresults are shown in Table I.

Example 4

[0090] A powder coating composition was prepared according to Example 1,except that 20.0 parts SCX 843 was used. The 60° gloss, pencil hardnessand MEK resistance of the cured powder coating were tested and the testresults are shown in Table I.

Example 5

[0091] A powder coating composition was prepared according to Example 1,except that 25.0 parts SCX 843 was used. The 60° gloss, pencil hardnessand MEK resistance of the cured powder coating were tested and the testresults in Table I. TABLE I MEK Ex. No. Gloss at 60° Pencil HardnessResistance 1 78 5 H Pass 2 67 5 H Pass 3 34 5 H Pass 4 20 5 H Pass 512.7 6 H Pass

Example 6-10

[0092] Powder coating compositions were prepared according to Example 1,except that 5.0 parts, 10.0 parts, 15.0 parts and 25.0 parts SCX 848 wasused, respectively, in each example. The 60° gloss, pencil hardness andMEK e of the cured powder coating were tested and the test results areTable II. TABLE II MEK Ex. No. Gloss at 60° Pencil Hardness Resistance 665 5 H Pass 7 33 5 H Pass 8 25 5 H Pass 9 11.4 5 H Pass 10  10.6 5 HPass

Example 11

[0093] A powder coating composition was prepared by pre-mixing 14.4parts SCX 848, 85.6 parts YD012, 1.0 parts 2-methyl-imidazole, 1.5 partsResiflow P-67 and 25 parts RCL-2 (white pigment). The mixture was thenmelt blended in a twin-screw extruder at 500 rpm, extruded, cooled,grounded and then sieved through 100 mesh sieve to obtain coatingpowders.

[0094] Using an electrostatic spray coating method, the above-preparedcoating powders were then applied onto a medium density fiberboard andcured in an oven at about 280° F. for about 15 minutes. The 60° gloss,pencil hardness and MEK resistance of the cured powder coating weretested and the test results are shown in Table III.

Example 12

[0095] A powder coating composition was prepared according to Example11, except that 15.6 parts SCX 848, 31.8 parts SCX 822, 52.6 partsGT7220 (Novolac epoxy resin, Vantico), 1.25 parts 2-phenyl-imidazoline,1.5 parts Modaflow 2000 and 30 parts RCL-2 were pre-mixed. The coatedmedium density fiberboard was cured in an oven at about 275° F. forabout 10 minutes. The 60° gloss and MEK resistance of the cured powdercoating were tested and the test results are shown in Table III.

Example 13

[0096] A powder coating composition was prepared according to Example12, except that 28.0 parts SCX 843, 72.0 parts GT7220, 1.5 parts2-phenyl-imidazoline, 1.5 parts Modaflow 2000 and 30 parts RCL-2 werepre-mixed. The 60° gloss and MEK resistance of the cured powder coatingwere tested and the test results are shown in Table III.

Example 14

[0097] A powder coating composition was prepared according to Example12, except that 28.7 parts by weight SCX 848, 71.3 parts by weightYD012, 2.0 parts 2-phenyl-imidazole, 1.5 parts Modaflow 2000 and 1.0parts BP120 (carbon black pigment) were pre-mixed. The 60° gloss and MEKresistance of the cured powder coating were tested and the test resultsare shown in Table III.

Example 15

[0098] A powder coating composition was prepared according to Example12, except that 41.1 parts SCX 848, 51.0 parts YD012, 7.7 parts TGIC,2.5 parts 2-phenyl-2-imidazoline/dodecandioic acid disalt, 1.5 partsModaflow 2000, 1.0 parts Lanco 1780 (wax) and 1.0 parts Taber Tiger 5512(abrasion resistant additive) were pre-mixed. The 60° gloss and MEKresistance of the cured powder coating were tested and the test resultsare shown in Table III. TABLE III MEK Ex. No. Gloss at 60° PencilHardness Resistance 11 30 3 H Pass 12 29 — Pass 13 11 — Pass 14 9 — Pass15 2 — Pass

Example 16

[0099] A powder coating composition was prepared by pre-mixing 19.0parts SCX 848, 39.3 parts YD012, 3.0 parts PT-810, 38.7 parts GMA 300,0.5 parts 2-phenyl-2-imidazoline/dodecandioic acid disalt, 2.0 parts2-phenyl-imidazole, 1.5 parts Resiflow P-67 and 30 parts RCL-2. Themixture was then melt blended in a twin-screw extruder at 500 rpm,extruded, cooled, grounded and then sieved through 100 mesh sieve toobtain coating powders.

[0100] Using an electrostatic spray coating method, the above-preparedcoating powders were then applied onto a medium density fiberboard andcured in an oven at about 280° F. for about 15 minutes. The 60° gloss,pencil hardness and MEK resistance of the cured powder coating weretested and the test results are shown in Table IV.

Example 17

[0101] A powder coating composition was prepared according to Example16, except that 1.0 parts 2-phenyl-2-imidazoline/dodecandioic aciddisalt was used. The 60° gloss, pencil hardness and MEK resistance ofthe cured powder coating were tested and the test results are shown inTable IV.

Example 18

[0102] A powder coating composition was prepared according to Example16, except that 2.0 parts 2-phenyl-2-imidazoline/dodecandioic aciddisalt was used. The 60° gloss, pencil hardness and MEK resistance ofthe cured powder coating were tested and the test results are shown inTable IV.

Example 19

[0103] A powder coating composition was prepared according to Example18, except that 1.0 parts 2-phenyl-imidazole was used. The 60° gloss,pencil hardness and MEK resistance of the cured powder coating weretested and the test results are shown in Table IV.

Example 20

[0104] A powder coating composition was prepared according to Example19, except that no PT-810 was used. The 60° gloss, pencil hardness andMEK resistance of the cured powder coating were tested and the testresults are shown in Table IV.

Example 21

[0105] A powder coating composition was prepared according to Example19, except that 1.0 parts PT-810 was used. The 60° gloss, pencilhardness and MEK resistance of the cured powder coating were tested andthe test results are shown in Table IV. TABLE IV MEK Ex. No. Gloss at60° Pencil Hardness Resistance 16 52 3 H Pass 17 41 3 H Pass 18 25 3 HPass 19 15 4-5 H Pass 20 10 7 H Pass 21 6.5 9 H Pass

Example 22

[0106] A powder coating composition was prepared by pre-mixing 28.7parts SCX 848, 71.3 parts YD012, 2.0 parts 2-phenyl-imidazole, 1.0 partsLanco 1780, 1.0 parts Taber Tiger 5512 (wax), 1.0 parts Ultranox 626(antioxidant) and 30 parts RCL-2. The mixture was then melt blended in atwin-screw extruder at 500 rpm, extruded, cooled, grounded and thensieved through 100 mesh sieve to obtain coating powders.

[0107] Using an electrostatic spray coating method, the above-preparedcoating powders were then applied onto a medium density fiberboard andcured in an oven at about 275° F. for about 10 minutes. The 60° gloss,pencil hardness and MEK resistance of the cured powder coating weretested and the test results are shown in Table V.

Example 23

[0108] A powder coating composition was prepared by pre-mixing 83.8parts SCX 835, 16.5 parts Epo-Thoto YDCN-500-90P (o-cresol novolac epoxyresin), 2.2 parts 2-phenyl-imidazole, 1.5 parts Modarez MFP A-25-P (flowcontrol additive, Synthron Inc., Morganton, WC), 0.29 parts Cab-O-SilCT111 G (fumed silica, Cabot Corp, Tuscola, Ill), and 30 parts RCL-2.The mixture was then melt blended in a twin-screw extruder at 500 rpm,extruded, cooled, grounded and then sieved through 100 mesh sieve toobtain coating powders.

[0109] Using an electrostatic spray coating method, the above-preparedcoating powders were then applied onto a medium density fiberboard andcured in an oven at about 275° F. for about 10 minutes. The 60° gloss,pencil hardness and MEK resistance of the cured powder coating weretested and the test results are shown in Table V. TABLE V MEK Ex. No.Gloss at 60° Pencil Hardness Resistance 22 15 4 H Pass 23 70 F Pass

Examples 24 and 25

[0110] Powder coating compositions were prepared by pre-mixing 46.3parts Epo-Thoto YD-012, 28.7 parts GMA 300, 10.0 parts Uniplex 280(sucrose benzoate), 1.5 parts Resiflow P-67, 1.5 parts2-phenylimidazole, and 30 parts of RCL-2. The mixture was then meltblended in a twin-screw extruder at 500 rpm, cooled, ground and sievedthrough 100 mesh sieve to obtain the component (A).

[0111] A premix of 100 parts of SCX 843, 30 parts of RCL-2 and 30 partsof Nyad 325 (wollastonite) was melt blended in a twin-screw extruder at500 rpm, cooled, ground and sieved through a 140 mesh sieve to obtainthe component (B).

[0112] Then the component (A) and component (B) were dry-blended at aw/w ratio of 100/10 and 100/15, respectively, to obtain two powdercoating compositions.

[0113] Using an electrostatic spray coating method, the above-preparedcoating powders were applied onto medium density fiberboard and cured inan oven at a part temperature of about 350° F. for about 3 minutes. The60° gloss, pencil hardness and MEK resistance of the cured powdercoatings were tested and the test results are shown in Table VI.

Example 26

[0114] A powder coating composition was prepared by pre-mixing 62.3parts Epo-Thoto YD-012, 5.2 parts EPO-THOTO YDC-1312, 20.5 parts GMA300, 12.0 parts SCX 843, 2.0 parts DYHARD PI, 1.0 part Resinflow P-67,30 parts RCL-2 and 0.266 parts Aluminum Oxide C. The mixture was thenmelt blended in a twin-screw extruder at 500 rpm, extruded, cooled,ground and then sieved through 140 mesh sieve to obtain coating powders.

[0115] Using an electrostatic spray coating method, the above-preparedcoating powders were then applied onto a medium density fiberboard andcured in an oven at a part temperature of about 350° F. for about 3minutes. The 60° gloss, pencil hardness and MEK resistance of the curedpowder coating were tested and the test results are shown in Table VI.

Example 27

[0116] A Powder coating was prepared according to example 26, exceptthat Epoxy Research Resin RSS-1407 was used instead of EPO-THOTOYDC-1312.

[0117] The 60° gloss, pencil hardness and MEK resistance of the curedpowder coating were tested and the test results are shown in Table VI.TABLE VI MEK Ex. No. Gloss at 60° Pencil Hardness Resistance 24 33 7 Hpass 25  7 9 H pass 26 25 H pass 27 40 2 H pass

We claim:
 1. A powder coating composition comprising: A). about 50% toabout 99% by weight of a first component comprising a-1) at least onepolyepoxy resin; and a-2) at least one catalyst, and B). about 1% toabout 50% by weight of a second component dry-blended with said firstcomponent (A), comprising at least one carboxylic acid functionalacrylic resin.
 2. The powder coating composition of claim 1, whereinsaid polyepoxy resin is selected from the group consisting ofnon-crystalline polyepoxy resins, crystalline polyepoxy resins, andmixtures thereof.
 3. The powder coating composition of claim 2, whereinsaid polyepoxy resin is a mixture of at least one non-crystallinepolyepoxy resin and at least one crystalline polyepoxy resin.
 4. Thepowder coating composition of claim 2, wherein said non-crystallinepolyepoxy resin is selected from the group consisting of glycidyl ethersof aromatic polyols, epoxy-functional acrylic resins, and mixturesthereof.
 5. The powder coating composition of claim 2, wherein saidcrystalline polyepoxy resin is selected from the group consisting oftriglycidyl isocyanurate (TGIC), diglycidyl ester of terephthalic acid,triglycidyl ester of trimellitic acid, tetramethylbisphenol diglycidylether, bisphenol S diglycidyl ether,2,5-di-t-butylbenzene-1,4-diglycidyl ether, 2,5-di-t-butylhydroquinonediglycidyl ether, diglycidyl isophthalate, epoxypropoxydimethylbenzylacrylamide, and mixtures thereof.
 6. The powder coating composition ofclaim 1, wherein said catalyst is selected from the group consisting ofimidazoles, imidazole/epoxy adducts, and mixtures thereof.
 7. The powdercoating composition of claim 1, wherein said second component furthercomprises at least one catalyst.
 8. The powder coating composition ofclaim 1, wherein either of said first component or second componentfurther comprises at least one ingredient selected from the groupconsisting of acid functional polyesters, fillers, slip additives,pigments, dyes, UV stabilizers, antioxidants, fluidizing agents, flowcontrol agents, degassing agents, flexibilizing agents, texturingagents, and mixtures thereof.
 9. The powder coating composition of claim1, wherein said composition provides a coating having 60° gloss of fromabout 2 to about
 95. 10. The powder coating composition of claim 9,wherein said composition provides a coating with 60° gloss of from about5 to about
 60. 11. The powder coating composition of claim 1, whereinsaid composition provides a coating having pencil hardness of greaterthan about F.
 12. The powder coating composition of claim 11, whereinsaid composition provides a coating having pencil hardness of greaterthan about 3H.
 13. A powder coating composition comprising: a). about 1%to about 50% by weight of at least one carboxylic acid functionalacrylic resin; b). about 50% to about 99% by weight of polyepoxy resins,of which from about 2% to about 15% by weight, based on the total epoxyresins, being at least one crystalline polyepoxy resin; and c). acatalyst in an effective amount to cure said composition.
 14. The powdercoating composition of claim 13, wherein said catalyst is selected fromthe group consisting of imidazoles, imidazole/epoxy adducts, tertiaryamines, imidazolines, imidazoline salts of mono- or di-carboxylic acids,tetraalkylammonium salts, phosphonium salts, and mixtures thereof. 15.The powder coating composition of claim 13, wherein said compositionprovides a coating having 60° gloss of from about 2 to about
 95. 16. Thepowder coating composition of claim 13, wherein said compositionprovides a coating with 60° gloss of from about 5 to about
 60. 17. Thepowder coating composition of claim 13, wherein said compositionprovides a coating having pencil hardness of higher than about 3H. 18.The powder coating composition of claim 13, wherein said crystallinepolyepoxy resin is selected from the group consisting of triglycidylisocyanurate (TGIC), diglycidyl ester of terephthalic acid, triglycidylester of trimellitic acid, tetramethylbisphenol diglycidyl ether,bisphenol S diglycidyl ether, 2,5-di-t-butylbenzene-1,4-diglycidylether, 2,5-di-t-butylhydroquinone diglycidyl ether, diglycidylisophthalate, epoxypropoxydimethylbenzyl acrylamide, and mixturesthereof.
 19. An article comprising a heat sensitive substrate, andcoated and cured on at least one surface of said substrate with a powdercoating composition of claim
 1. 20. An article comprising a heatsensitive substrate, and coated and cured on at least one surface ofsaid substrate with a powder coating composition of claim 13.